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| United States Patent |
5,057,602
|
|
Gabara
, et al.
|
October 15, 1991
|
Para-phenylene diamine polymer color improvement with sequestering agent
Abstract
A process is provided for increasing the brightness and decreasing the
discoloration of PPD polymer during the polymer production. Sequestering
agents having a stability constant (K.sub.1), for Fe(III), greater than
about 12 are slurried with the PPD polymer in water and then rinsed,
neutralized, and dried.
| Inventors:
|
Gabara; Vlodek (Richmond, VA);
Lapallo; Angela M. (Richmond, VA)
|
| Assignee:
|
E. I. DuPont de Nemours and Company (Wilmington, DE)
|
| Appl. No.:
|
431227 |
| Filed:
|
November 3, 1989 |
| Current U.S. Class: |
528/486; 528/348; 528/492; 528/499 |
| Intern'l Class: |
C08G 069/46 |
| Field of Search: |
528/486,492,499
|
References Cited
U.S. Patent Documents
| 3475771 | Nov., 1969 | Quynn | 528/486.
|
Primary Examiner: Anderson; Harold D.
Claims
What is claimed is:
1. A process for treating para-phenylene diamine (PPD) polymer comprising
the steps of:
(a) combining PPD polymer, newly-isolated from a PPD polymer polymerization
system, at least 0.1%, based on weight of the PPD polymer, of a
sequestering agent having a stability constant (K.sub.1), for Fe(III), of
greater than 12, and water in an amount adequate to yield a slurry of the
PPD polymer;
(b) agitating the combination for at least ten seconds;
(c) separating the PPD polymer from the combination.
2. The process of claim 1 wherein the PPD polymer is poly(p-phenylene
terephthalamide) (PPD-T).
3. The process of claim 1 comprising additional steps of:
(d) combining the separated PPD polymer with water in an amount adequate to
yield a slurry of the PPD polymer;
(e) agitating the combination for at least ten seconds;
(f) separating the PPD polymer from the combination;
(g) optionally, repeating steps (d), (e), and (f).
4. The process of claim 1 wherein the newly-isolated PPD polymer is
combined before it has been exposed to a pH greater than about 2.
5. A process for treating poly(p-phenylene terephthalamide) (PPD-T)
comprising the steps of:
(a) combining PPD-T acid crumb, at least 0.1%, based on weight of the
PPD-T, of a sequestering agent having a stability constant (K.sub.1), for
Fe(III), of greater than 12, and water in an amount adequate to yield a
slurry of the PPD-T;
(b) agitating the combination for at least ten seconds;
(c) separating the PPD-T from the combination.
6. The process of claim 5 wherein the acid crumb is combined before it has
been exposed to a pH greater that about 2.
7. The process of claim 5 wherein the sequestering agent is an
aminocarboxylic acid.
8. The process of claim 7 wherein the aminocarboxylic acid is
ethylenediamine tetraacetic acid.
9. The process of claim 7 wherein the aminocarboxylic acid is
nitrilotriacetic acid.
10. The process of claim 5 wherein the water is in an amount of 5 to 50
times the weight of the PPD-T.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
Para-phenylene diamine (PPD) polymer is subject to an excessive and, until
now, uncontrolled and unpredictable discoloration in isolation of the
polymer from low-temperature solution polymerization. Discolored polymer
yields discolored products and, even if the discoloration is not, of
itself, unacceptable, discoloration of inconsistent degrees may well be
unacceptable. This invention relates to a process for preparing PPD
polymer having an improved color uniformity from run to run.
2. Description of the Prior Art
Prior to the present invention, it has been the practice to isolate PPD
polymer from the solution polymerization media by washing in water without
particular regard to the wash water or materials contained therein.
U.S. Pat. No. 4,308,374, issued Dec. 29, 1981 on the application of
Vollbracht et al. discloses that poly-p-phenylene terephthalamide is
isolated by mixing the reaction mixture with water or aqueous alkali.
There is no mention of treatment to control discoloration in the polymer
product.
U.S. Pat. Nos. 3,819,587 and 3,869,429, issued June 25, 1974 and Mar. 4,
1975 on the applications of Kwolek and Blades, respectively, disclose that
polyamides are isolated by being vigorously stirred with water and then
filtered. Those patents, further, state that one of the washes may contain
sodium carbonate or hydroxide. Neither patent is concerned with polymer
discoloration.
Japanese Application Publication (Kokai) 58-40,306, published Mar. 9, 1983
discloses addition of alkali metal or ammonium salt of
ethyenediaminetetraacetic acid to water-soluble acrylic copolymers to
maintain the polymer water-solubility during pulverization. There is no
mention of polymer discoloration.
U.S. Pat. No. 4,650,624, issued Mar. 17, 1987 on the application of Reinehr
et al., discloses that EDTA can be used to increase the whiteness of dry
spun acrylic fibers. The EDTA is added to the spinning solution and has
nothing to do with production of the acrylic polymer.
U.S. Pat. No. 3,415,611, issued Dec. 10, 1968 on the application of
Callahan et al., discloses that EDTA can be used to increase the whiteness
of wet spun acrylonitrile fibers. The EDTA is added to the spinning
solution or to spun fibers in the gel state and has nothing to do with
production of the acrylonitrile polymer.
SUMMARY OF THE INVENTION
The present invention provides a process for preparing PPD polymer of
reduced discoloration wherein polymer isolation from the solution
polymerization medium includes treatment with a sequestering agent. The
sequestering agent can be any aminocarboxylic acid having a stability
constant (K.sub.1), for Fe(III), of greater than about 12; and the
treatment should include addition of the sequestering agent to the
unisolated PPD polymer before the pH of the polymer system has been raised
above 2. The PPD polymer which is most usually used in practice of this
invention is poly(p-phenylene terephthalamide) (PPD-T).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graphical representation of Color L (Brightness) versus the
concentration of sequestering agent used for treating PPD-T by the process
of this invention.
FIG. 2 is a graphical representation of Color L (Brightness) versus the pH
of the treatment system at constant concentration of sequestering agent to
show that practice of the process of this invention is substantially
independent of pH below a pH of about 8 or 9.
DETAILED DESCRIPTION OF THE INVENTION
Discoloration of PPD polymer during the polymer production, has been a
continuing problem. At some times, there have been excessive discoloration
and from time-to-time in the same process there have been varying degrees
of discoloration. It has been desired to minimize the discoloration and to
make the degree of color uniform over a long course of polymer production.
Although the reason for the effect is not entirely understood, it has now
been discovered that discoloration of PPD polymer can be decreased and
controlled by addition of particular sequestering agents to the polymer
system prior to final washing and drying. The sequestering agents can be
added to PPD polymer newly-isolated from the polymerization system before
neutralization, that is, before the PPD polymer has been exposed to a pH
greater than about 2, and, preferably before a pH greater than about 1.
The sequestering agent could be added to the polymerization system before
the polymerization is completed if it doesn't interfere with the
polymerization reaction.
By PPD polymer is meant any polymer resulting from mole-for-mole
polymerization of p-phenylene diamine with a coreactant and, also,
copolymers resulting from incorporation of other diamines with the
p-phenylene diamine. As a general rule, other diamines can be used in
amounts up to as much as about 50 mole percent of the total diamine. It is
believed that discoloration of PPD polymers is caused by impurities in the
PPD or generated by reaction of the PPD; and that the discoloration can be
decreased by the process of the present invention in polymers having at
least 25 total mole percent PPD.
The present invention has been most often used with PPD-T. By PPD-T is
meant the homopolymer resulting from mole-for-mole polymerization of
p-phenylene diamine and terephthaloyl chloride and, also, copolymers
resulting from incorporation of small amounts of other diamines with the
p-phenylene diamine and of small amounts of other diacid chlorides with
the terephthaloyl chloride. As a general rule, other diamines and other
diacid chlorides can be used in amounts up to as much as about 10 mole
percent of the p-phenylene diamine or the terephthaloyl chloride, or
perhaps slightly higher, provided only that the other diamines and diacid
chlorides have no reactive groups which interfere with the polymerization
reaction. PPD-T, also, means copolymers resulting from incorporation of
other aromatic diamines and other aromatic diacid chlorides such as, for
example, 2,6-naphthaloyl chloride or choroor dichloroterephthaloyl
chloride; provided, only, that the other aromatic diamines and aromatic
diacid chlorides be present in amounts which permit preparation of
anisotropic spin dopes. Preparation of PPD-T is described in U.S. Pat.
Nos. 3,869,429; 4,308,374; and 4,698,414.
PPD polymer, for treatment by the process of this invention, is prepared by
polymerization in a liquid medium and is isolated from the polymerization
system by washing with water. The treatment process of this invention can
be conducted in the polymerization system or in any of the washes. The
process is preferably conducted in the first wash, at first contact of the
polymer with water and before the pH of the system is greater than 2 and,
preferably, before the pH is greater that 1. The polymerization system for
PPD-T generally yields a product which is a granular, solid, mass with a
dry appearance; but containing only about 5-15% PPD-T. That polymer
product is named herein, and is well-known by workers in this art to be,
"acid crumb".
In conduct of the process of this invention, the acid crumb is combined
with water in an amount adequate to yield an aqueous slurry of the
polymer. Water is usually used in an amount 5 to 50 times the weight of
the polymer in the crumb and with the sequestering agent in an appropriate
amount. The combination of PPD polymer, sequestering agent, and water is
agitated for a time adequate to permit contact of substantially all of the
PPD polymer with the sequestering agent. A time of as little as 10 seconds
of agitation has been deemed to be adequate for systems having efficient
agitation. The process of this invention can be conducted in a batch
manner or as a continuous process; care being taken, in the case of a
continuous process, to maintain the amounts of the various component
streams within the appropriate ranges.
Sequestering agents useful in the practice of this invention include
compounds having stability constants, for Fe(III), of greater than about
12. Sequestering agents which are particularly useful are aminocarboxylic
acids having K.sub.1 >12 for Fe(III). Particularly preferred are
ethylenediamine tetraacetic acid (EDTA) having K.sub.1 =25.1 and
nitrilotriacetic acid (NTA) having K.sub.1 =15.9 for Fe(III). It has been
concluded that sequestering agents having low stability constants are not
effective in this invention. For example, it has been found that citric
acid having K.sub.1 =11.4 for Fe(III) and sodium tripolyphosphate having
K.sub.1 =3.6 for Fe(III) cannot be used to practice this invention.
The benefits of this invention are realized by addition of any amount of
sequestering agent. Very small amounts of the agent may yield small
improvement. Generally, good results are obtained by use of about 0.25 to
0.75% of the sequestering agent based on the weight of PPD polymer to be
treated. As stated, less of the agent can be used for diminished effect.
Noticeable improvement in brightness can be obtained by as little as 0.1%
EDTA based on weight of PPD polymer. More than 0.75% of the sequestering
agent can be used; but amounts in excess of about 0.75% do not appear to
provide substantial additional benefit. The brightness of PPD polymer
treated by 3.0% sequestering agent is about the same as PPD polymer
treated by 0.75% of the same agent.
Color determinations are made using a Hunter Lab Tristimulus Colorimeter
model D25M-9 such as that sold by Hunterlab of Reston, Va., U.S.A.
Brightness of polymer for this invention was determined by measuring the
"L" color component. The "L" color component is a measure of the blackness
or whiteness of a polymer sample on a scale from 0 (black) to 100 (white).
For the purposes of this invention, the absolute values of the "L" value
determined from the test are not important. The differences in color
between treated and untreated polymers are easily seen by relative values
from the treated polymers to the controls. Color improvements resulting
from practice of this invention can be determined using any sensitive
colorimeter to compare treated and untreated polymers.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
EXAMPLE 1
This example describes practice of the process of this invention using EDTA
and PPD-T.
PPD-T was made in accordance with the teachings of U.S. Pat. No. 4,308,374
to yield an acid crumb of about 11% polymer and having a dry appearance.
Three hundred weight parts of the acid crumb were mixed with 500 weight
parts of distilled water to form a slurry. The pH of the slurry was about
0.6. It was estimated that the filter cake contained about 33 weight parts
of PPD-T. To the slurry of PPD-T, still being mixed, was added 0.25 weight
parts of EDTA sequestering agent (0.75%, based on the weight of the
PPD-T). The pH of the system was adjusted to 6.0 using sodium hydroxide
solution, and the system was agitated for about ten more minutes.
After mixing the polymer with the sequestering agent, the system was
filtered and the filter cake was slurried and filtered four more times
using about 750 weight parts of water each time. Finally, the filter cake
was dried in an oven for about 16 hours at about 140.degree. C.
As a control, the above procedure was followed omitting addition of the
sequestering agent.
Color determinations were conducted on the products of this example in
accordance with the color determination test method described above. It
should be noted that color differences of only two units on the color
scale are easily detected by the unaided eye. The color of the polymer
treated by this invention was 78.0 and the color of the control was 68.2.
EXAMPLE 2
In this example, the procedure of Example 1 was followed with the exception
that different amounts of sequestering agent were added to the slurry in
different runs. Results of the several runs are shown in the the table,
below, and are, also, depicted in the graphical representation of FIG. 1.
The column headed ".DELTA.L" shows the color difference between the
treated PPD-T and the Control PPD-T. Note that the runs of this example
were conducted on polymer made on three different occasions and that, as a
consequence, the color of the controls is slightly different from batch to
batch.
______________________________________
Batch % sequestering agent
Color L .DELTA.L
______________________________________
I None (control) 73.2 --
0.03 75.1 1.9
0.75 78.6 5.4
3.00 78.5 5.3
II None (control) 68.2 --
0.15 76.5 8.3
0.30 75.8 7.6
0.45 76.9 8.7
0.75 76.8 8.6
3.00 77.1 8.9
III None (control) 68.0 --
0.03 69.1 1.1
0.06 71.9 3.9
0.09 72.1 4.1
0.12 76.7 8.7
0.15 70.8 2.8
0.75 78.1 10.1
0.75 80.3 12.3
______________________________________
EXAMPLE 3
In this example, the procedure of Example 1 was followed with the exception
that the pH of the initial slurry was adjusted to have different values
for different runs. Results of the several runs are shown in the table,
below, and are, also, depicted in the graphical representation of FIG. 2.
The amount of sequestering agent was maintained at 0.75%, based on polymer
weight. The control was PPD-T maintained at pH 6 with no sequestering
agent treatment.
______________________________________
pH Color L
______________________________________
6 (control) 69.0
1 82.2
3 81.6
4 79.4
5 80.8
6 80.9
7 81.6
8 80.7
9 78.6
10 78.0
11 76.7
______________________________________
EXAMPLE 4
In this example, the procedure of Example 1 was followed with the exception
that different sequestering agents were used for different runs. Results
of the runs are shown in the table, below. The amount of sequestering
agent was maintained at 0.75%, based polymer weight.
______________________________________
Sequestering Agent Color L L
______________________________________
None (control) 69.0 --
Na.sub.2 EDTA* 78.6 9.6
Na.sub.2 NTA** 78.6 9.6
Na.sub.3 Citric Acid***
71.9 2.9
Sodium Tripolyphosphate
71.4 2.4
______________________________________
*disodium salt of EDTA
**disodium salt of NTA
***trisodium salt of Citric Acid
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